1. [Field of the Invention]
The present invention relates to a torque detector for detecting torque when external force is applied to a rotation shaft without contact in a car power steering unit or the like.
2. [Description of the Prior Art]
In a car power steering unit, torque applied to a steering wheel must be detected to determine the amount of assist force. There is known a torque detector disclosed by Japanese Laid-open Patent Application No. 63-65331 as an example of the torque sensor used for this purpose of the prior art. The structure of this device will be described with reference to FIGS. 10 to 12.
In these figures, reference symbol 1A denotes a shaft to be measured, 10 and 11 a pair of electrode bases fixed to the shaft 1A, 12 and 13 electrodes fixed to the electrode bases 10 and 11, respectively, 16 a rotary transformer consisting of a rotary core 16a and a stationary core 16b, 17a and 18a cores embedded in the rotary core 16a, and 17b and 18b cores embedded in the stationary core 16b. As shown in FIG. 11, the electrode 12 consists of electrodes 12a to 12d and the electrode 13 consists of electrodes 13a to 13b, all of which are shaped like a fan with the center of the shaft 1A as the center thereof. An overlapped portion between the electrodes 12a and 13a form a capacitor C1, an overlapped portion between the electrodes 12b and 13a forms a capacitor C2, an overlapped portion between the electrodes 12c and 13b form a capacitor C3, and an overlapped portion between the electrodes 12d and 13b form a capacitor C4.
These capacitors are connected to one another to form a Wheatstone bridge circuit as shown in FIG. 12. The terminals xe2x80x9ccxe2x80x9d and xe2x80x9cdxe2x80x9d of the Wheatstone bridge circuit are connected to both ends of the coil 17a of the rotary transformer 16 and the terminals xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are connected to both ends of the coil 18a of the rotary transformer 16. The coils 17b and 18b magnetically connected to the coils 17a and 18a are connected to an unshown detection circuit.
A description is subsequently given of the operation of the torque detector. When torque is applied to the shaft 1A from the steering wheel, the torsion deformation of the shaft 1A occurs and relative torsion displacement between the pair of electrode bases 12 and 13 occurs. For example, when the electrodes 13a and 13b displace in a clockwise direction with respect to the electrodes 12a to 12d in FIG. 11, the areas of the overlapped portions change, whereby the capacitance of each of the capacitors C1 and C3 decreases and the capacitance of each of the capacitors C2 and C4 increases. Since the Wheatstone bridge circuit is thereby imbalanced, an AC voltage eO is generated between the terminals xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d by supplying an AC voltage eB between the terminals xe2x80x9ccxe2x80x9d and xe2x80x9cdxe2x80x9d. Since this voltage is proportional to the amount of torsion deformation of the shaft and the torque applied to the shaft, the torque is obtained from the voltage between the terminals xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d.
Since the terminals xe2x80x9caxe2x80x9d to xe2x80x9cdxe2x80x9d are connected to the detection circuit by the rotary transformer 16 without contact, a signal can be transmitted even when the shaft 1A turns.
Since the torque detector of the prior art is constituted as described above, to transmit a signal to the fixed detection circuit from a detection unit provided in the turning shaft 1A, the rotary transformer must be used as in the case above or a slip ring must be used in other known examples with the result that the torque detector becomes complex in structure.
In other torque detector of the prior art, a movable magnetic member is displaced by the twisting of a torsion bar or the shaft caused by torque and this displacement is obtained as a change in the inductance of a coil wound round the shaft.
Since the magnetic characteristics of the magnetic member such as a yoke or movable magnetic member used in the rotary transformer or the coil have temperature characteristics, a torque detection error is easily produced.
It is an object of the present invention which has been made to solve the above problems of the prior art to provide a torque detector having a simple structure and high accuracy.
According to a first aspect of the present invention, there is provided a torque detector for detecting torque which is applied between a first shaft and a second shaft arranged coaxial to each other in such a manner that one end of the first shaft faces one end of the second shaft, wherein the torque detector comprises an elastic member for connecting the first shaft to the second shaft and generating torsion displacement between the first shaft and the second shaft according to the torque between the first and second shafts, a ring-shaped movable electrode attached to the shafts so that it displaces in an axial direction according to a relative torsion angle between the first and second shafts, a fixed electrode installed at a location where its surface is opposed to the surface of the movable electrode and it does not turn together with the shafts, and detecting means for detecting capacitance between the fixed electrode and the movable electrode.
According to a second aspect of the present invention, there is provided a torque detector which comprises first and second fixed electrodes arranged on the same side of the movable electrode and detecting means for detecting capacitance between each of the first fixed electrode and the second fixed electrode and the movable electrode.
According to a third aspect of the present invention, there is provided a torque sensor which comprises a third fixed electrode arranged on a side opposite to the first and second fixed electrodes of the movable electrode.
The above elastic member has a first group of elastic columns which are inclined at a predetermined angle with respect to the first shaft and formed at predetermined intervals on a side where it is fixed to the first shaft and a second group of elastic columns which are inclined at the same predetermined angle in an opposite direction to that of the first group of elastic columns with respect to the second shaft and formed at predetermined intervals on a side where it is fixed to the second shaft. The movable electrode is supported between the first group of elastic columns and the second group of elastic columns directly or indirectly.
The above and other objects, features and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.